2-mercapto-1, 3, 4-oxadiazoles as antifoggants



United States Patent 2-MERCAPTO-1,3,4-0XADIAZOLES AS ANTIFOGGANTS Charles F. H. Alien and John J. Sagura, Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application May 23, 1956 Serial No. 586,654

8 Claims. (Cl. 96-409) This invention relates to fog-inhibiting agents and stabilizers for photographic emulsions, and to photographic emulsions containing them.

It is well known that photographic emulsions on storage tend to lose sensitivity and-to become spontaneously developable without exposure to light. There is normally a detectable amount of the silver salt reduced during development in the areas where no exposure was given; this is commonly called fog," and sometimes called chemical fog where it is necessary to distinguish between it and the effects of accidental exposure to radiation; in this invention, we are not concerned with the latter.

Fog depends both on the emulsion and the conditions of development; for a given emulsion it increases with the degree of development. With constant development conditions, it tends to increase with time, temperature and relative humidity of storage conditions; it is common practice to make accelerated tests of the stability of photographic emulsions by storage at increased temperature or humidity, or both. It is, of course, desirable to have emulsions as stable as possible under the conditions of high temperature and humidity which may occur in tropical climates, for example. Fog usually appears over the whole area of the sensitive coating, but when severe, it frequently is non-uniform. Fog may also be caused by exposure to chemicals, for example, hydrogen sulfide and other reactive sulfur compounds, hydrogen peroxide vapor, and strongly reducing materials. While antifoggants and stabilizers may protect, to some extent, against such effects, it is normally understood that an antifoggant protects against spontaneous growth of fog during prolonged storage or storage at high temperatures and humidities, or during development to maximum contrast and speed, or both.

It is, accordingly, an object of our invention to provide a method for stabilizing photographic emulsions. A further object of our invention is to maintain the sensitivity and fog of silver halide emulsions at or close to initial optimum values under keeping conditions of high temperature and humidity. A further object is to provide photographic silver halide emulsions containing antifoggants or stabilizers. Other objects will become apparent from a consideration of the following description and examples.

The above objects are accomplished by adding to the photographic silver halide emulsion a compound selected from those represented by the following general formula:

I. N--N wherein R represents a monocyclic aryl group of the benzene series, such as phenyl, o-, mand p-tolyl, o-, mand p-chlorophenyl, m and p-nitrophenyl, o-, mand p-methoxyphenyl, 0-, mand p-ethoxyphenyl, etc. (e. g., a monocyclic aryl group of the benzene series containing from 6 to 8 carbon atoms).

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The fog inhibitors which we propose to use are added to the emulsion during the process of manufacture, to avoid loss of sensitivity and to inhibit the growth of fog with passage of time under non-ideal conditions of storage.

A solution of the compounds of the invention when added in suitable concentration, before coating, to unsensitized, chemically sensitized, or optically sensitized photographic emulsions does not appreciably affect the sensitometric values for sensitivity and fog when measurements are made soon after coating. When sensitometric measurements are made at appreciable intervals of time, at elevated temperatures and dry or somewhat humid conditions, these compounds do stabilize photographic speed and maintain fog at a low level.

The preparation of silver halide emulsions involves three separate operations: (1) the emulsification and digestion or ripening of the silver halide, (2) the freeing of the emulsion from excess soluble salts, usually by washing, and (3) the second digestion or after-ripening to obtain increased sensitivity. (Mees The Theory of the Photographic Process, (1942).) We prefer to add the fog-inhibiting agents after the final digestion or afterripening, although they can advantageously be added prior to digestion.

Listed below are a number of compounds coming within the scope of the above general formula, which we have found to be particularly advantageous in practicing our invention. These compounds belong to a group of compounds which we shall designate as oxadiazoles.

CtH l i l-SH 2-mercapto-5-pheny1-1,3,4-oxadtazole.

N N i 2-mercapto-5- (m-nitrophenyl) -1,3,4-oxadiazole i o o-sH 2-mereapto-5- (o-rnethoxyphenyl) -1,3,4oxa.diazole The above compounds can be prepared according to methods previously described in the prior art, see for example, Jour. Chem. 806., 1952, pages 4811-17.

The photographic emulsions used in practicing our invention are generally of the developing-out type; also, it is to be understood that photographic emulsions of varying halide content can advantageously be used. The antifoggant compounds used in our invention have been found particularly useful when employed in conjunction with gelatino-silver bromiodide emulsions, although they can also be advantageously employed for stabilizing other silver halide emulsions, such as gelatino-silver-chloride, bromide, chlorobromide, chlorobromiodide, etc.

The emulsions can also be chemically sensitized by any of the accepted procedures. The emulsions can be digested with, naturally active gelatin, or sulfur compounds can be added such as those described in Sheppard U. S.

3 Patent 1,574,944 and U. S. 1,623,499, and Sheppard and Brigham U. S. Patent 2,410,689.

The emulsions can also be treated with salts of the noble metals such as ruthenium, rhodium, palladium, iridium and platinum, all of which belong to group VIII of the periodic table of elements and have an atomic weight greater than 100. Representative compounds are ammonium chloropalladate, potassium chloroplatinate and sodium chloropalladite, which are used for sensitizing in amounts below that which produces any substantial fog inhibition, as described in Smith and Trivelli U. S. Patent 2,448,060, and as antifoggants in higher amounts, as described in Trivelli and Smith U. S. Patents 2,566,245 and 2,566,263.

The emulsions can also be chemically sensitized with gold salts as described in Waller and Dodd U. S. Patent 2,399,083, or stabilized with gold salts as described in Damschroder U. S. Patent 2,597,856 and Yutzy and Leermakers U. S. Patent 2,597,915. Suitable compounds are potassium chloroaurite, potassium aurithiocyanate, potassium chloroaurate, auric trichloride and 2- aurosulfobenzothiazole methochloride.

The emulsions can also be chemically sensitized with reducing agents such as stannous salts (Carroll U. S. Patent 2,487,850), polyamines such as diethylene triamine (Lowe and Jones U. S. Patent 2,518,698), polyamines, such as spermine (Lowe and Allen U. S. Patent 2,521,925), or bis-(fi-aminoethyl) sulfide and its watersoluble salts (Lowe and Jones U. S. Patent 2,521,926).

The emulsions can also be stabilized with the mercury compounds of Allen, Byers and Murray U. S. Patent 2,728,663, Carroll and Murray U. S. Patent 2,728,664, and Leubner and Murray U. S. Patent 2,728,665.

The stabilizing combinations of oxadiazoles are effective in the presence or absence of optical sensitizing dyes. Since optical sensitizing may affect stability of emulsions with respect to sensitivity, fog and latent image changes, the action of the compounds of this invention is not completely independent of optical sensitizing or other emulsion variables. We have found, however, that both un-sensitized emulsions and emulsions sensitized with cyanine or merocyanine dyes, or both, can be treated with oxadiazoles according to our invention.

The antifoggant and stabilizing action was determined by incubation of the emulsions for one week at a temperature of 120 F. and constant relative humidity (obtained by placing the emulsions in closed containers, the ambient temperature being about 70 F. and relative humidity about 55 percent prior to sealing the containers).

The efficiency of the various antifoggants was determined by measuring the speed, gamma and fog of the incubated emulsions containing an antifoggant and comparing these measurements with those of the same batch of emulsion before incubation. ments were made with a photographic emulsion containing no antifoggant both before and after incubation.

The tests were made using high speed, negative-type silver bromiodide emulsions (coated on cellulose acetate supports), which had been panchromatically sensitized with a cyanine dye and chemically sensitized with sulfur and gold compounds. The test films were exposed on an intensity scale sensitometer and developed for 5 minutes in a developer having the following composition:

Water to make 1 liter.

The speed, gamma and fog for each of the emulsion coatings were then measured as indicated above. The speed figure shown in the following table is on a logarith- Also, similar measuremic scale, obtained by the formula: 100(1 minus log E), where E is the exposure in meter-candle-seconds of sunlight quantity required to produce a density of 0.30 above fog. The results obtained are given in the following table.

Fresh Coatings Incubated Coatings Example Gom/poulnd g. mo AgX) Speed Gamma Fog Speed Gamma Fog 1 (a) Control 312 1. 16 17 299 1.02 30 (b) 1 003) 312 1. 11 15 309 1.01 22 312 1. l3 16 308 1.01 22 313 1. 21 15 311 20 312 1. 11 15 309 99 22 310 1. 21 15 310 1. 07 22 In a manner similar to that illustrated in the above example, other oxadiazoles selected from those represented by the above general Formula I can be incorporated in photographic emulsions for the purpose of stabilization. The fog-inhibiting agents useful in practicing our invention can be used in various kinds of photographic emulsions. In addition to being useful in ordinary non-sensitized emulsions, they can also be used in orthochromatic, panchromatic and X-ray emulsions. If used with sensitizing dyes, they can be added to the emulsion before or after the dyes are added. Suitable dispersing agents for the silver halide emulsions stabilized according to our invention comprise gelatin, or other colloids, such as collodion, albumen, cellulose organic derivatives, synthetic resins, etc.

The optimum amount of fog-inhibiting agent can be determined by making the customary tests employed in the emulsion making. Of course, the optimum amount for a given emulsion will vary depending on the presence of emulsion addenda, such as chemical sensitizers, optical sensitizers, etc. In general, we have found that from 0.0001 to 1.0 g. of fog-inhibiting agent per mole of silver halide is sufiicient for the purposes of our invention.

*Instead of adding the fog-inhibiting agent directly to the photographic emulsion, it is sometimes desirable to incorporate the fog-inhibiting agent in a separate layer which is placed in contact with the silver halide emulsion layer which is to be stabilized. Under such conditions, of course, it is advisable to use a higher concentration of fog-inhibiting agent than indicated above. The antifoggants of our invention function advantageously in acid or alkaline photographic silver halide emulsions. In alkaline emulsions, Water-soluble salts of the compounds of Formula I above are formed to some extent, although this action does not deleteriously affect the antifoggant action of these compounds.

We have also found that compounds corresponding to those of Formula I above wherein the oxygen atom is replaced by a sulfur atom (i. 'e., thiadiazoles) can also be used to stabilize photographic silver halide emulsions against the formation of fog.

The compounds represented by Formula I above can be employed not only in conventional photographic silver halide emulsions as described above, but they can be employed in other image-forming layers, such as those customarily used in solvent-transfer processes, wherein the image-receiving layer containing a compound of Formula I is placed in contact with an image-transferring layer, 1such as a silver halide emulsion dispersed in a stripping 'ayer.

What we claim as our invention and desire secured by Letters Patent of the United States is:

1. A photographic silver halide emulsion containing a compound selected from those represented by the following general formula:

wherein R represents a monocyclic aryl group of the benzene series.

2. A photographic gelatino-nlver halide developing-out emulsion containing a compound selected from those represented by the following general formula:

II R-C C-SH wherein R represents a monocyclic aryl group of the benzene series containing from 6 to 8 carbon atoms.

3. A photographic gelatino-silver 'bromiodide developing-out emulsion containing a compound selected from those represented by the following general formula:

II II n-o 0-sn 0 wherein R represents a monocyclic aryl group of the benzene series containing from 6 to 8 carbon atoms. 4. A photographic silver halide emulsion containing a compound represented by the following formula:

5. A photographic silver halide emulsion containing a compound represented by the following formula:

G R /G-SH N--N Clancy) an 8. A photographic silver halide emulsion containing a 5 compound represented by the following formula:

NN J; 25-811 OCHs I References (Iited in the file of this patent UNITED STATES PATENTS 2,432,864 Dimsdale et al. Dec. 16, 1947 

1. A PHOTOGRAPHIC SILVER HALIDE EMULSION CONTAINING A COMPOUND SELECTED FROM THOSE REPRESENTED BY THE FOLLOWING GENERAL FORMULA: 